A&A 370, L5–L8 (2001) Astronomy DOI: 10.1051/0004-6361:20010297 & c ESO 2001 Astrophysics The radio surroundings of the microquasar GRO J1655–40 J. A. Combi1,G.E.Romero1, P. Benaglia1, and I. F. Mirabel2,3 1 Instituto Argentino de Radioastronom´ıa, C.C.5, (1894) Villa Elisa, Buenos Aires, Argentina 2 CEA/DMS/DAPNIA/Service D’Astrophysique, Centre d’Etudes´ de Saclay, 91191 Gif-sur-Yvette, France 3 Instituto de Astronom´ıa y F´ısica del Espacio (IAFE), C.C. 67, Suc. 28, Buenos Aires, Argentina Received 12 February 2001 / Accepted 22 February 2001 Abstract. We report the results of a study of the radio surroundings of the superluminal microquasar GRO J1655−40. We have searched for extended continuum structures that might be indicative of the pres- ence of a supernova remnant (SNR) associated with the formation of the compact object in the binary system. We also carried out HI-line observations of the region looking for a local minimum created by an explosive event. Our results indicate that there is, in fact, a bubble in the large-scale HI distribution around GRO J1655−40. We suggest that this structure might be created by the original supernova explosion occurred a few hundred thousand years ago and whose signatures can be traced by the overabundance of α-elements recently found by Israelian et al. (1999) in the companion star. Key words. X-rays: stars – stars: evolution – ISM: bubbles – ISM: supernova remnants 1. Introduction source in the radio continuum. If the age of the system is less than 106 years, these observational features might be The soft X-ray transient GRO J1655−40 is one of the best detectable. black hole candidates in the Galaxy (Mirabel & Rodriguez 1999). It is a low mass X-ray binary whose compact ob- ject has a mass in the range 5.5−7.9 M (Shahbaz et al. 2. Data analysis and new observations 1999). The system is located at 3.2 kpc from the Earth and presents frequent superluminal ejections at radio wave- 2.1. Radio continuum lengths (e.g. Tingay et al. 1995; Hjellming & Rupen 1995). We have used data from the 4.85-GHz PMN survey Recently, Israelian et al. (1999) found convincing (Condon et al. 1993) in order to produce a large-scale evidence of a supernova origin for the black hole in map (∼1◦ × 1◦) of the surroundings of GRO J1655−40. − GRO J1655 40. They detected a clear overabundance of Although these data are optimized for sizescales of less α-elements in the secondary star, whose internal tempera- than 40 arcmin, the good resolution and sensitivity of tures are not high enough as to synthesize them. These el- the images render them a valuable tool to search for ements seem to have been produced in a massive compan- SNR candidates, especially if they are not of large an- − ion (with 25 40 M ) which exploded both contaminating gular size (Duncan et al. 1997). For this work, we apply the secondary star with matter enriched with nucleosyn- to the 4.85-GHz data an additional filtering process in or- thetic products and creating the black hole. Israelian et al. der to remove the galactic diffuse emission on scales larger (1999) estimated that the explosion should have occurred 6 than 30 arcmin (see Combi et al. 1998 for details of the less than 10 years ago. Gaussian filtering method). The resulting map is shown In this Letter, we present the results of a study of in Fig. 1, along with a 1.4-GHz image of the small-scale − the radio environment of GRO J1655 40. Our aim is emission of the inner region obtained from the NVSS Sky to detect any existent signature of an explosive event Survey (Condon et al. 1998) at a better resolution. These in this region. A supernova explosion should have intro- latter observations were obtained with the Very Large duced modifications in the ISM around the binary system, Array (VLA) in the compact D and DnC configurations. displacing material and creating a low density bubble. Additionally, the expanding shock front should accelerate electrons up to relativistic energies, producing a shell-type 2.2. HI observations Send offprint requests to:J.A.Combi, We have performed HI observations towards the area e-mail: [email protected] of interest with a 30-m single dish telescope located at Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20010297 L6 J. A. Combi et al.: Radio surroundings of GRO J1655−40 Table 1. Point radio sources near GRO J1655−40 Source (l, b) F1.4GHz ID (deg, deg) (mJy) S1 (344.87◦,+2.62◦) 24.9 – S2 (344.92◦,+2.54◦) 12.8 – S3 (344.96◦,+2.46◦)5.43 – S4 (344.98◦,+2.42◦)4.48 – S5 (345.02◦,+2.38◦) 37.7 – S6 (345.03◦,+2.49◦)5.83 – S7 (345.10◦,+2.37◦) 10.6 PMN J1654-3949 3. Main results Figure 1 (upper panel) shows the radio continuum im- age at 4.85 GHz of the surroundings of GRO J1655−40. Asmall(∼240 ×240) semi-circular radio source, which very much resembles a SNR, can be clearly seen in the center of the image. This extended, partial shell source is centered at (l, b) ≈ (345.14◦, +2.48◦). It has an integrated flux den- sity of ∼0.5 Jy at 4.85 GHz, although uncertainties due to the background filtering make difficult to estimate a realistic value. The microquasar is located at a distance of ∼0.15 deg from the center of the radio source. Figure 1 (lower panel) shows the VLA image at 1.4 GHz (Condon et al. 1998) of the vicinity of GRO J1655−40. The microquasar was undetectable at the epoch of the observations and its position is marked with a Fig. 1. Upper panel: filtered radio emission at 4.85 GHz of cross. There are seven point sources near the microquasar the region surrounding GRO J1655−40. The position of the with fluxes above 5 mJy. They have been labeled from microquasar is marked by a cross. Contours are labeled in steps S1 to S7 in the image (notice that the SNR candidate is of 10 mJy beam−1, starting from 35 mJy beam−1. Lower panel: not visible in this map because these data are optimized small-scale, higher resolution, VLA image at 1.4 GHz for the for point source detection). The bright, compact source region close to GRO J1655−40. Radio contours are in steps located at (l, b) ∼ 345.1◦,+2.37◦ is identified in the PMN − − of 2 mJy beam 1 starting from 1 mJy beam 1. The arrows catalogue as PMN J1654-3949. The sources S1 to S6 have indicate the directions of the observed superluminal ejections no entry in any point source catalog at present. The mea- sured characteristics of all these sources are summarized in Table 1. the Instituto Argentino de Radioastronom´ıa (IAR), Villa Figure 2 clearly shows the development of a cavity − − Elisa, Argentina. The observations were carried out during around GRO J1655 40 in the HI maps from 42 to − −1 four consecutive sessions on November 20–23, 2000. The 30 km s . Although the cavity do not form a shell due receiver is a helium-cooled HEMT amplifier with a 1008- to the steep density gradient towards the galactic plane, channel autocorrelator at the backend. System parameters the existence of a local minimum in the HI distribution and additional details of the observational technique can at the position of the microquasar is evident. This kind of be found in Combi et al. (1998). The HI line was observed “horse-shoe” morphology has been observed around early- in hybrid total power mode and the sky was sampled on type stars with very strong stellar winds (e.g. Benaglia & a0.35◦ rectangular grid. Each grid position was observed Cappa 1999), where a strong energy release in a highly during 60 s with a velocity resolution of ∼1kms−1 and a inhomogeneous medium has occurred. coverage of 450 km s−1. A set of HI brightness temper- In Fig. 3 we present the integrated column density map −1 ature maps (∆Trms ∼ 0.2 K) were made for the velocity for the velocity interval from −40 to −30 km s . Standard interval ∆v =(−80 km s−1, +20 km s−1). Those maps for galactic rotation models (Fich et al. 1989) indicate that the interval −42 to −30 km s−1, corresponding to the HI this HI hole is located at a kinematic distance of 3 to distribution at a distance centered at ∼3.5 kpc, are shown 4 kpc, in good accordance with the estimated distance to in Fig. 2. GRO J1655−40. J. A. Combi et al.: Radio surroundings of GRO J1655−40 L7 Fig. 3. Integrated column density map for the velocity range from −40 to −30 km s−1. Contour labels are in units of 1019 cm−2. Microquasar position indicated with a cross source could be a background SNR or, perhaps, an artifact formed by several close and unresolved point sources. At larger sizescales there are not other clear continuum structures that could be associated with a SNR. We have inspected larger maps, of ∼6◦ × 6◦, in search of additional Fig. 2. HI brightness temperature channel maps (contour signatures, but nothing was found. This is a very confused labels in K) obtained for the velocity range from −42 to region located nearly towards the Galactic Center and − −1 − 30 km s around the position of GRO J1655 40 (indicated the detection of weak continuum features with low sur- with a cross).